Variable optical dispersion compensator

ABSTRACT

There will be provided a dispersion compensator which has hardly group delay ripples in the broad bandwidth, applicable to the wavelength multiplex transmission system. The structure is arranged such that an interferometer, the reflection factor of one surface of which is nearly 100%, and a mirror are arranged in parallel or at a slight angle, and light emerging from the first collimator becomes incident on the second collimator after alternately reflected two or more times between the interferometer and the mirror. Further, an interferometer, at least one of the reflection factor or the thickness of which has been changed in the longitudinal direction, is caused to slide in the longitudinal direction or the temperature of the interferometer is changed to thereby make the amount of dispersion variable. Further, these dispersion compensation units are provided in multistage to realize a variable dispersion compensator which has hardly group delay ripples in the broad bandwidth.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical dispersion compensator suitable for applying to an optical transmission system using optical fibers or a system in which an optical transmission scheme based on wavelength multiplexing has been adopted.

[0003] 2. Description of Related Art

[0004] In recent years, a study and commercialization of a long distance optical transmission system for using an optical amplifier as a repeater have been increased. In order to cope with multi media service with a particular emphasis on internet, a trend toward larger capacity due to WDM (Wavelength Division Multiplex) for multiplexing multiple signal light having different wavelength to one piece of optical fiber which is a communication transmission path has been considered as an effective technique. In such a long distance optical transmission system, the transmission speed or transmission distance is restricted by large amounts by a phenomenon referred to as wavelength dispersion of the optical fiber. The wavelength dispersion is a phenomenon in which light having different wavelength propagates through the optical fiber at different speeds. Since an optical spectrum of an optical signal modulated at high speeds includes different wavelength components, these components reach a receiver at different time under the influence of the wavelength dispersion when propagating through the optical fibers. As a result, distortion occurs in the optical signal waveform after the fiber transmission. In order to suppress waveform degradation by such a dispersion, a technique referred to as dispersion compensation becomes important. The dispersion compensation is a technique for canceling out the wavelength dispersion characteristic of the optical fibers to prevent the waveform from being degraded by arranging an optical element having the inverse of the wavelength dispersion characteristic of the optical fibers for use with the transmission path in an optical transmitter, a receiver, a repeater or the like. As such an optical element, that is, a dispersion compensator, devices having an inverted dispersion characteristic such as dispersion compensation fibers and optical fiber Bragg grating have been studied and commercialized.

[0005] Dispersion tolerance indicates a range of residual dispersion (total sum of amounts of dispersion by transmission path fibers and the dispersion compensator) that satisfies a certain reference of transmission quality. Since the dispersion tolerance becomes smaller inversely with the square of bit rate of an optical signal, the dispersion compensation technique becomes further important as the transmission speed rises. For example, in a 10 Gbps transmission system, when it is taken into account that the dispersion tolerance of the optical signal is nearly 1000 ps/nm, and that the amount of dispersion of a single mode fiber is about 17 ps/nm/km, the system will be able to transmit only about 60 km without the aid of the dispersion compensation technique. On the other hand, the dispersion tolerance in a 40 Gbps transmission is about 60 ps/nm that corresponds to this {fraction (1/16)}, and this corresponds to about 4 km in the single mode fiber. Although at present, the transmission distance of the main line stem optical fiber transmission using the optical repeater is from several tens kilometers to about thousands kilometers, it is necessary to change the amount of dispersion of the dispersion compensator in accordance with the transmission distance. For example, in the 10 Gbps transmission system, there has been adopted a method for preparing for a dispersion compensator of a fixed amount of compensation in increments of 100 ps to about several hundreds ps in advance in consideration of the dispersion tolerance, and determining the amount of compensation in accordance with the transmission distance at the time of installation for installation, or the like. In this case, for a dispersion compensator, there is adopted a representative method for using a dispersion compensation fiber having wavelength dispersion with an inverse symbol to the transmission path. Next, in a 40 Gbps transmission system, a dispersion compensator whose amount of compensation dispersion is capable of varying in increments of 10 ps to about several tens ps similarly is considered necessary. Moreover, in this case, variations in amount of wavelength dispersion due to temperature of the transmission path fibers cannot be ignored. For this reason, a dispersion compensator capable of controlling the amount of dispersion in a variable manner becomes necessary.

[0006] However, these conventional dispersion compensators have also various problems. When dispersion compensating a fixed amount, since as huge compensation fibers as several kilometers to hundreds kilometers are required in the dispersion compensation fibers, storage space for the fibers becomes large. Also, in order to compensate for a loss of the dispersion compensation fibers, there is the possibility that extra light amplifiers will be required. Further, the dispersion compensation fibers have generally small mode field diameter, and there is the possibility that they cause a great fiber non-linear effect and distortion of transmission waveform.

[0007] In the case of an optical fiber Bragg grating, the compensation characteristic greatly changes depending upon a slight change in wavelength because there exist ripples on wavelength in the transmission characteristic and wavelength dispersion characteristic. Accordingly, it is known that the transmission characteristic when used for the dispersion compensation is inferior to the dispersion compensation fibers. Also, those which are large in the amount of dispersion and wavelength bandwidth are difficult to be fabricated from a production viewpoint, and there is a problem that those having a narrow bandwidth require stabilization in temperature and wavelength. Also, in the dispersion compensation fibers, the amount of dispersion cannot be continuously made variable in terms of principles, but it is difficult to realize such a variable dispersion compensation as the amount of dispersion is continuously changed in accordance with the change in the amount of dispersion in the transmission path.

SUMMARY OF THE INVENTION

[0008] In the case of the optical fiber Bragg grating, as a method for realizing the continuous variable dispersion compensation, there has been reported, for example, a method for producing a chirped grating by giving a temperature gradient in the longitudinal direction of the optical fiber Bragg grating for dispersion compensation transmission. In this case, by controlling the temperature gradient, it becomes possible to dispersion compensate for a variable amount. However, this method is difficult to obtain a uniform temperature gradient, and has problems in which sufficient dispersion compensation of performance cannot be made, such as ripples occurring in the wavelength dispersion. This has a problem in practicality. It is an object of the present invention to solve such a problem as described above and to provide a dispersion compensator which has hardly ripples in the broad bandwidth.

[0009] The structure is arranged such that an interferometer, the reflection factor of one surface of which is nearly 100%, and a mirror are arranged in parallel or at a slight angle, whereby light emerging from the first collimator becomes incident on the second collimator after resonance and emission are repeated two or more times by the interferometer. Further, through the use of an interferometer, at least one of the reflection factor or the thickness of which has been changed in the longitudinal direction, the interferometer is caused to slide or its temperature is changed by a heater or the like, whereby there is provided an arrangement for making the amount of dispersion variable for the dispersion compensator. Further, these dispersion compensators are provided in multistage, whereby it becomes possible to realize a variable dispersion compensator applicable to the wavelength multiplex optical transmission system whose ripples have been restrained in the broad bandwidth. For more detailed embodiments, please refer to the column, “DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.”

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a view showing an interferometer which is a basic element of an optical dispersion compensator according to an embodiment of the present invention;

[0011]FIG. 2 is a view showing group delay characteristic of an interferometer which is a basic element of an optical dispersion compensator according to an embodiment of the present invention;

[0012]FIG. 3 is a view showing first structure of the optical dispersion compensator according to an embodiment of the present invention;

[0013]FIG. 4 is a view for explaining in detail the first structure of the optical dispersion compensator according to the embodiment of the present invention;

[0014]FIG. 5 is a view showing the group delay characteristic of the first structure of the optical dispersion compensator according to the embodiment of the present invention;

[0015]FIG. 6 is a view showing basic structure using an interferometer which is a basic element of an optical dispersion compensator according to the embodiment of the present invention;

[0016]FIG. 7 is a view showing the first structure of the optical variable dispersion compensator according to an embodiment of the present invention;

[0017]FIG. 8 is a view showing second structure of the optical variable dispersion compensator according to the embodiment of the present invention;

[0018]FIG. 9 is a view showing the second structure of the optical dispersion compensator according to the embodiment of the present invention;

[0019]FIG. 10 is a view showing a third structure of the optical dispersion compensator according to the embodiment of the present invention;

[0020]FIG. 11 is a view showing second structure of the optical variable dispersion compensator according to the embodiment of the present invention;

[0021]FIG. 12 is a view showing fourth structure of the optical dispersion compensator according to the embodiment of the present invention;

[0022]FIG. 13 is a view showing third structure of the optical variable dispersion compensator according to the embodiment of the present invention;

[0023]FIG. 14 is a view showing an example of first group delay characteristic of an optical variable dispersion compensator according to the embodiment of the present invention;

[0024]FIG. 15 is a view showing fourth structure of the optical variable dispersion compensator according to the embodiment of the present invention;

[0025]FIG. 16 is a view showing fifth structure of the optical variable dispersion compensator according to the embodiment of the present invention;

[0026]FIG. 17 is a view showing an example of second group delay characteristic of an optical variable dispersion compensator according to an embodiment of the present invention;

[0027]FIG. 18 is a view showing an example of wavelength dispersion characteristic of an optical variable dispersion compensator according to an embodiment of the present invention;

[0028]FIG. 19 is a view showing sixth structure of the optical variable dispersion compensator according to the embodiment of the present invention; and

[0029]FIG. 20 is a view showing an example of third group delay characteristic of an optical variable dispersion compensator according to the embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Hereinafter, with reference to the drawings, the detailed description will be made of embodiments of the present invention. First, Gires Tournois (GT) interferometer will be described. FIG. 1 shows the structure of the interferometer. The interferometer is obtained by coating both surfaces of a plane plate 10 which have precisely been made parallel with each other with a reflected film 11, 12. For the reflected film, metal film made of gold, silver or the like, having high reflection factor, or dielectric multilayer film or the like is used. Particularly, an interferometer having the reflection factor of one side being 100% is referred to as GT interferometer. In the case of such an interferometer, since its transmission factor is constant irrespective of wavelength, it is referred to as all pass filter. As regards phase (group delay time), however, it has wavelength dependence. Group delay time τ at this time is represented by the following expression. $\begin{matrix} {\tau = \frac{\Delta \quad {L\left( {1 - r^{2}} \right)}}{1 + r^{2} + {2\quad r\quad {\cos \left( {{\omega \quad \Delta \quad T} + \varphi_{0}} \right)}}}} & {{Expression}\quad 1} \end{matrix}$

[0031] where τ is an amplitude reflection factor, {overscore (ω)} is each frequency of light, φ₀ is an optical phase change caused in reflected film, and ΔL is an optical distance caused by going and returning along the parallel plane plate. FIG. 2 shows group delay characteristics when the amplitude reflection factor τ is set to 80% and 40%. Wavelength dispersion D is obtained by differentiating the group delay time τ with respect to the wavelength. $\begin{matrix} {D = \frac{\tau}{\lambda}} & {{Expression}\quad 2} \end{matrix}$

[0032] This is equal to the inclination of the curve of FIG. 2. When dispersion of higher order (referred to also as wavelength differentiation of dispersion or dispersion slope) is ignored, the variable dispersion compensator has preferably as flat characteristic as possible. In other words, a place where the inclination shown in FIG. 2 is linear within as wide a range as possible can be used, and this portion will be referred to as an effective bandwidth. It can be seen that in the case of higher reflection factor than this (r=80%), the effective bandwidth becomes narrower although the amount of dispersion (the inclination in the figure) is large, and in the case of lower reflection factor (r=40%), the effective bandwidth becomes wider although the amount of dispersion is small.

[0033] In a dispersion compensator particularly in the 40 Gbps long distance optical transmission system, however, it becomes important for a high-speed signal that the effective bandwidth is broad (40 GHz or higher), and that a large variable range can be taken. This time, there has been invented a method for expanding the effective bandwidth with a few optical parts by reflecting two or more times through the use of a mirror. With reference to FIG. 3, the description will be made of the basic structure of a dispersion compensator according to the present invention. A mirror 20 is arranged in parallel with or at a slight angle with respect to the interferometer. Speaking in details, the mirror 20 is obtained by coating one side of the substrate with a reflected film 21 with a high reflection factor. In order to explain the group delay characteristic by a mathematical expression, the description will be made with reference to FIG. 4. A collimator 30 is arranged at an angle of incidence of θ₀ slightly obliquely from the vertical direction with respect to the mirror 20. The collimator is an optical part which emits light for propagating through an optical fiber into the space as a parallel beam. Light emerging from the collimator 30 is reflected by a mirror 21, emerges after resonating with the interferometer (10, 11, 12), and after these processes are repeated two or more times, becomes incident into the collimator 31. At this time, when the positional relationship between the interferometer (10, 11, 12) and the mirror 20 is such that an angle Δθ is given from the parallel, the angle of incidence θ with respect to the interferometer becomes θ=θ₀+θ, θ₀+3Δθ, . . . . When the angle of incidence with respect to the interferometer becomes large, the optical path length which resonates also becomes large, the peak of the group delay is shifted to the long wavelength side, and at the same time, the reflection factor also lowers. When total group delay characteristic τ_(total) is enciphered, it is represented as follows. $\begin{matrix} {\tau_{total} = {{\sum\limits_{i = 1}^{k}\quad \tau_{i}} = {\sum\limits_{i = 1}^{k}\left\lbrack \frac{\Delta \quad {L\left( {1 - {r_{i}}^{2}} \right)}}{1 + r_{i}^{2} + {2\quad r_{i}{\cos \left( {{\omega \quad \Delta \quad L} + \varphi_{0}} \right)}}} \right\rbrack}}} & {{Expression}\quad 3} \\ {{{\Delta \quad L} = \frac{2\quad n\quad L}{c\quad \cos \quad \Theta}}\quad} & {{Expression}\quad 4} \end{matrix}$

[0034] where Θ is an angle of incidence in the interferometer, and is represented by n sin Θ=sin θ. FIG. 5 shows the group delay characteristic at each reflection and their total τ_(total), that is, the group delay characteristic in the structure shown in FIG. 3. Although the dispersion is represented by wavelength differentiation of group delay, it can be seen that the structure proposed can be expanded rather than the effective bandwidth to be given by at a single resonance, that is, such an interferometer as a single unit as shown in FIG. 6. In the structure shown in FIG. 6, the optical dispersion compensator is composed of: collimators 30 and 31; a plane plate 10; and reflected films 11 and 12. An optical beam emerging from the collimator 30 resonates only once with an interferometer composed of the plane plate 10 and the reflected films 11 and 12, and thereafter, becomes incident on the collimator 31. FIGS. 3 and 5 show a case of Δθ≢0, but we can safely say that Δθ=0. In this case, since there is no effect of the peak wavelength of group delay being shifted, the amount of dispersion can be taken large although the bandwidth is not expanded. Thereby, it becomes possible to design a dispersion compensator having an arbitrary amount of dispersion or bandwidth. Thus, it becomes possible to expand the bandwidth of the dispersion compensator and to restrain ripples. Further, a method for making the amount of dispersion compensation variable on the basis of this method is shown in FIG. 7. In this case, there is used a GT interferometer in which a different group delay characteristic can be obtained by changing the reflection factor and thickness in the longitudinal direction. In the GT interferometer (11, 12), a GT interferometer, the reflection factor of the reflected film 12 of which is as high (100% ideally) as possible is adopted, and the reflection factor of the reflected film-11 is caused to change in the longitudinal direction. As regards a method for changing the reflection factor, there are a method for changing the thickness of the coating, a method for changing the composition of the coating little by little, and the like. By sliding the GT interferometer (11, 12) in a direction in which the reflection factor has been changed as described above, there can be provided a mechanism for making the amount of dispersion compensation variable. In order to slide the GT interferometer, a fixed base 60 has a guide rail 61, which guides in a direction that the GT interferometer (11, 12) operates. Also, the fixed base 60 fixes the collimator 30, 31 and the reflected film 21. In order that a beam emerging from the collimator 30 becomes incident on the collimator 31, the GT interferometer (11, 12) becomes capable of sliding along the guide rail 61 by means such as a stepping motor without changing the direction of the beam. As another method of the variable mechanism, it is also conceivable to change the temperature of the interferometer by adding a heater 40 as shown in FIG. 8. In this case, the coefficient of thermal expansion of the interferometer substrate changes and the wavelength that resonates slightly changes. This corresponds to a shift of the peak wavelength of group delay shown in FIGS. 2 and 5. As a matter of course, as the variable mechanism, both a method for sliding the interferometer and a method for changing by means of a heater may be combined. In this respect, the dispersion compensator using such a CT interferometer shows wavelength dependence in terms of group delay and dispersion characteristic. This period is referred to as Free Spectral Range (FSR), and is 100 GHz (about 0.8 nm) in this figure. It becomes particularly effective in the wavelength multiplex transmission to have such a wavelength periodicity. If FSR is made equal to a wavelength space during wavelength multiplex transmission, the CT interferometer will be able to give similar effects with respect to signals with all wavelength. Accordingly, such a variable dispersion compensator is exceedingly effective in the wavelength multiplex transmission system.

[0035] Further, as a method for expanding the bandwidth and restraining ripples, it is conceivable to combine CT interferometers having two or more kinds of different reflection factors for reflection. The structure for this arrangement is shown in FIG. 9, and the structure is arranged such that second CT interferometer (13, 14, 15) is provided in place of the mirror (20, 21) in FIG. 3. The variable dispersion compensator of FIG. 9 is composed of: collimators 30 and 31; a first CT interferometer consisting of a plane plate 10 and reflected films 11 and 12; and a second CT interferometer consisting of a plane plate 13 and reflected films 14 and 15. An optical beam emerging from the collimator 30 becomes incident on the collimator 31 after resonation and emission are alternately repeated by means of two GT interferometers several times. In this figure, the first GT interferometer (10, 11, 12) and the second GT interferometer (13, 14, 15) have been arranged to oppose to each other at a slight angle, but it may be possible to cause these to oppose to each other in parallel as described above. When they are arranged at a slight angle as described above, the optical beam emerging from the collimator 30 repeats resonance while changing the angle of incidence on the GT interferometer little by little as shown. Thereby, a dispersion compensator which has hardly group delay ripples in the broad bandwidth as described above can be realized.

[0036] Also, another structure in which two types of GT interferometers have been arranged is shown in FIG. 10. In this structure, two types of GT interferometers (10, 11, 12) and (13, 14, 15) have been arranged at an angle Δθ₁ and Δθ₂ respectively from the parallel with respect to the mirror (20, 21). By doing so, it is possible to independently set the position of the peak that occurs in those two GT interferometers and a change in the reflection factor, and to further enlarge the degree of freedom in designing concerning the expanding bandwidth, ripples and amount of dispersion. The variable dispersion compensator shown in FIG. 10 is composed of: collimators 30 and 31; a first GT interferometer consisting of a plane plate 10 and reflected films 11 and 12; a second GT interferometer consisting of a plane plate 13 and reflected films 14 and 15; and a mirror consisting of a substrate 20 and a reflected film 21. The optical beam emerging from the collimator 30 resonates with the first GT interferometer for emission, and is reflected by the mirror. After this process is repeated several time, the optical beam becomes incident on the collimator 31. In this figure, the first GT interferometer (10, 11, 12), the second GT interferometer (13, 14, 15) and the mirror (20, 21) have been arranged to oppose with a slight angle, but it may be possible to cause these to oppose in parallel as described above. When arranged at a slight angle as described above, the optical beam emerging from the collimator 30 repeats resonance while changing the angle of incidence on those two GT interferometers little by little as shown. Thereby, a dispersion compensator which has further hardly group delay ripples in the broad bandwidth can be realized.

[0037] Next, structure for providing a variable mechanism is shown in FIG. 11. The variable dispersion compensator of FIG. 11 is composed of: collimators 30 and 31; a first GT interferometer consisting of reflected films 11 and 12; a second GT interferometer consisting of reflected films 13 and 14; a fixedbase 50 for fixing those two interferometers; a mirror consisting of a reflected film 21; and a fixed base 60 having a guide rail 61. In this case, the reflected films 11 and 13 changes the reflection factor in a direction (here, referred to as a longitudinal direction) perpendicular to a direction that the optical beam emerging from the collimator 30 advances as indicated by light and shade of the figure while repeating resonance and reflection with the mirror and those two interferometers. Those two GT interferometers has their angles fixed by the fixed base 50, and are slid at the same time in the direction indicated in the figure, whereby the amount of dispersion compensation can be made variable. In order to cause these to slide, the fixed base 60 has the guide rail 61 to guide in a direction that those two GT interferometers operate. Also, the fixed base 60 has fixed the collimators 30, 31 and the reflected film 21. In order that a beam emerging from the collimator 30 becomes incident on the collimator 31, those two GT interferometers become capable of sliding along the guide rail 61 by means such as a stepping motor without changing the direction of the beam. With such structure, the variable mechanism of a dispersion compensator which has hardly group delay ripples in the broad bandwidth can be realized. In the case of this structure, however, the peak wavelength of group delay wh ich those two types of GT interferometers have must satisfy the broad bandwidth dispersion characteristic. The peak wavelength of group delay is determined by the angle of incidence on the interferometer, the substrate reflection factor, an amount of phase change of light due to reflection on the coating and the like. We hope that the manufacture could be performed in such a manner that all of these amounts conforms to the design, but if not, the yields will be reduced. For this reason, in order to independently set the angle of incidence on the interferometer with respect to two types of GT interferometers, such structure that a second mirror shown in FIG. 12 is provided between the GT interferometers is conceivable. The variable dispersion compensator shown in FIG. 12 is composed of: collimators 30 and 31; a first GT interferometer consisting of a plane plate 10, and reflected films 11 and 12; a second GT interferometer consisting of a plane plate 13 and reflected films 14 and 15; a first mirror consisting of a substrate 20 and a reflected film 21; and a second mirror consisting of a substrate 22 and a reflected film 23. The optical beam emerging from the collimator 30 resonates and emits with the first GT interferometer (10, 11, 12), and is reflected by the first mirror (20, 21). This process is repeated several time, the angle of incidence is adjusted by the second mirror (22, 23), and the optical beam resonates and emits with the second GT interferometer (13, 14, 15), is reflected by the first mirror (20, 21), and after this process is repeated several time, becomes incident on the collimator 31. In this figure, the GT interferometer (10, 11, 12), the GT interferometer (13, 14, 15) and the mirror (22, 23) are given a slight angle with respect to the first mirror (20, 21), and further these are arranged to oppose at the respective different angles, but it may be possible to cause these to oppose in parallel as described above. When a slight angle is given as described above, the optical beam emerging from the collimator 30 repeats the resonance while changing the angles of incidence on those two GT interferometers little by little as shown, whereby the dispersion compensation which has further hardly group delay ripples in the broad bandwidth can be realized.

[0038] Next, structure for providing the variable mechanism is shown in FIG. 13. The variable dispersion compensator of FIG. 13 is composed of: collimators 30 and 31; a first GT interferometer consisting of reflected films 11 and 12; a second GT interferometer consisting of reflected films 13 and 14; a fixed base 50 for fixing those two interferometers; a first mirror consisting of a reflected film 21; a second mirror consisting of a reflected film 23; and a fixed base 60 having a guide rail 61. In this case, the reflected films 11 and 13 change the reflection factor and the thickness in a direction perpendicular to a direction that the optical beam emerging from the collimator 30 advances as indicated by light and shade of the figure while repeating resonance and reflection with the mirror and those two interferometers, that is, in the longitudinal direction. The second mirror consisting of those two GT interferometers and the reflected film 23 has its angle fixed by the fixed base 50, and is caused to slide at the same time in the direction indicated in the figure, whereby the amount of dispersion compensation can be made variable. In order to cause it to slide, the fixed base 60 has the guide rail 61 to guide a direction that the second mirror consisting of those two GT interferometers and the reflected film 23 operates. Also, the fixed base 60 has fixed the collimators 30, 31 and the reflected film 21. In order that a beam emerging from the collimator 30 becomes incident on the collimator 31, the second mirror consisting of those two GT interferometers and the reflected film 23 becomes capable of sliding along the guide rail 61 by means such as a stepping motor without changing the direction of the beam. With such structure, the variable mechanism of a dispersion compensator which has hardly group delay ripples in the broad bandwidth can be realized.

[0039] With such structure, results obtained by measuring the group delay characteristics by experiment are shown in FIG. 14. Two kinds of interferometers used in this experiment are those having amplitude reflection factor r=40% and 16%. By doing so, characteristics of effective bandwidth 53 GHz, dispersion 123 ps/nm, and ripples 1.9 ps or less have been obtained. An effect of the effective bandwidth being expanded through the use of two kinds of interferometers is shown in FIG. 20. FIG. 20 shows respective group delay caused when resonance has been made five times with the first interferometer 1, respective group delay caused when resonance has been made five times with the second interferometer 2 and their total. In this figure, the peak positions of those two interferometers have been adjusted so as to expand the effective bandwidth. Usually, in the WDM transmission system, there is used optical wavelength (ITU grid) instituted by ITU (International Telecommunication Union) that is an International Organization for Standardization. For this reason, in order to take the effective bandwidth of the dispersion compensator wide and to match the center wavelength with the ITU grid, the angles of incidence of those two interferometers must be independently set. For this setting, the existence of the second mirror consisting of a reflected film 23 is required. Also, FIG. 14 shows group delay characteristic when the reflection factors of the interferometers are changed by calculation at the same time. From this result, a dispersion compensator of the amount of variable dispersion −300 ps/nm to +300 ps/nm could be realized.

[0040] As the structure of the variable dispersion compensator, such structure as shown in FIG. 15 is also conceivable. In this structure, the variable dispersion compensator is composed of: an input port 100; a plus-side variable dispersion compensation unit 120; a minus-side variable dispersion compensation unit 130; and an output port 110. The plus-side variable dispersion compensation unit 120 is a dispersion compensator which gives an amount of dispersion compensation to the plus side from 0. Also, the minus-side variable dispersion compensation unit 130 is a dispersion compensator which gives an amount of dispersion compensation to the minus side from 0. For example, when a plus amount of dispersion compensation should be given to the dispersion compensator, the amount of dispersion compensation to be given by the minus-side variable dispersion compensation unit 130 is set to 0, and the plus-side amount of dispersion compensation is changed by the plus-side variable dispersion compensation unit 120. Conversely, when a minus amount of dispersion compensation should be given to the dispersion compensator, the amount of dispersion compensation to be given by the plus-side variable dispersion compensation unit 120 is set to 0, and the plus-side amount of dispersion compensation is changed by the minus-side variable dispersion compensation unit 130.

[0041] Since a dispersion compensator according to the present invention has wavelength periodicity of FSR as described above, it has been explained that it is possible to collectively dispersion compensate at the time of wavelength multiplex transmission. However, this FSR is slightly shifted together with the wavelength space, whereby it becomes also possible to slightly change the amount of dispersion compensation with respect to a signal for each wavelength. Thereby, it also becomes possible to compensate for the dispersion of higher order (dispersion slope). Here, the dispersion of higher order D₂ is an amount to be represented by the following expression obtained by further differentiating the dispersion with respect to the wavelength. $\begin{matrix} {D_{2} = \frac{D}{\lambda}} & {{Expression}\quad 5} \end{matrix}$

[0042] The structure of a dispersion compensator capable of compensating for up to such a dispersion of higher order is shown in FIG. 16. In this structure, the dispersion compensator is composed of: an input port 100; the plus-side variable dispersion compensation unit 120; the minus-side variable dispersion compensation unit 130; a plus-side variable higher-order dispersion compensation unit 140; a minus-side variable higher-order dispersion compensation unit 150; and an output port 110.

[0043] With reference to FIGS. 17 and 18, the description will be made of the dispersion and an example of the compensator of higher-order dispersion.

[0044]FIG. 17 shows a case where the wavelength dependence of the group delay characteristic becomes nearly a secondary function (parabola), and solid line indicates positive curvature, that is, positive dispersion slope, while dotted line indicates negative curvature, that is, negative dispersion slope. Further, through the use of the Numerical Formula (2), the wavelength dependence of the amount of dispersion at this time is shown in FIG. 18. Similarly to FIG. 17, the solid line indicates the positive dispersion slope and the dotted line indicates the negative dispersion slope. As the variable mechanism, the reflection factor and the thickness are changed in the longitudinal direction as described above, or by causing the temperature to change through the use of a heater or the like, the amount of dispersion slope can be changed. Also, if there are a pair of higher order dispersion compensators having positive and negative dispersion slopes at the same absolute value, it will be possible to make into a dispersion compensator which has further hardly group delay ripples by canceling the higher order dispersions each other out. Its example will be described with reference to FIG. 19. In this structure, the higher order dispersion compensator is composed of: an input port 100; a plus-side variable higher-order dispersion compensation unit 140; a minus-side variable higher-order dispersion compensation unit 150; and an output port 110. For example, when temperature change due to a heater or the like is used as a variable mechanism of the higher order dispersion compensation unit, the peak of dispersion shown in FIG. 18 shifts together with the temperature. When shifted in the direction of the arrow shown in FIG. 18 by the temperature change, the amount of dispersion as the dispersion compensator becomes positive, and when shifted in a direction opposite to the arrow, the amount of dispersion as the dispersion compensator becomes negative. The closer the positions of mutual peaks come, the narrower the effective bandwidth becomes, but it becomes possible to increase the absolute value of the amount of dispersion. With such structure, it becomes possible to realize a variable dispersion compensator which has hardly group delay ripples.

[0045] As described above, according to the embodiment of the present invention, it is possible to collectively dispersion compensate for each signal light at the time of wavelength multiplex transmission, and to expand the transmission distance in the transmission system. Further, even as regards the higher order dispersion, the dispersion compensation can be performed. Thus, through the use of the dispersion compensator according to the present invention, a single and low-priced optical communication system excellent in transmission characteristic can be constructed. 

What is claimed is:
 1. An optical dispersion compensator, wherein a second plane of a flat plate-shaped interferometer having first and second planes which oppose each other and a reflected surface of a mirror having one reflected surface are arranged to oppose in parallel, or said second plane and said reflected surface are arranged obliquely and to oppose in such a manner that an imaginary plane including said second plane and an imaginary plane including said reflected surface intersect in a line or at a point; and wherein a reflection factor of said first plane is higher than that of said second plane.
 2. The optical dispersion compensator according to claim 1, wherein a reflection factor of said first plane is 90% and over to 100% incl. and the reflection factor of said second plane is 0% and over to 90% incl., wherein the reflection factor of said mirror is 90% and over to 100% incl., and wherein the structure is arranged such that an optical beam is caused to become incident from between said second plane and said reflected plane, said optical beam is alternately reflected once, or two or more times between said interferometer and said mirror, and thereafter, said optical beam is caused to emit from between said second plane and said reflected plane, whereby one or multiple signal light having different wavelength is given wavelength dispersion.
 3. The optical dispersion compensator according to claim 2, wherein the structure is arranged such that said second plane of said interferometer has a square shape, that at least one of the reflection factor of said second plane of said interferometer or thickness of said interferometer is changed in a direction of a side of said square, and that said interferometer is movable in parallel in said direction.
 4. The optical dispersion compensator according to claim 2, wherein there is provided temperature adjusting means for causing a change in temperature to said interferometer adjacent to said interferometer.
 5. The optical dispersion compensator according to claim 3, wherein for said mirror, there are 2n pieces of mirrors (n is a natural number) and for said interferometer, there are 2n pieces of interferometers, and wherein n pieces of plus variable dispersion compensation units having a positive variable dispersion compensation function and n pieces of minus variable dispersion compensation units having a negative variable dispersion compensation function are connected in multistage.
 6. The optical dispersion compensator according to claim 4, wherein for said mirror, there are 2n pieces of mirrors (n is a natural number) and for said interferometer, there are 2n pieces of interferometers, and wherein n pieces of plus variable higher order dispersion compensation units having a positive variable higher order dispersion compensation function and n pieces of minus variable higher order dispersion compensation units having a negative variable higher order dispersion compensation function are connected in multistage.
 7. The optical dispersion compensator according to claim 1, wherein there are a plurality of said interferometers: a first interferometer and a second interferometer; said first and second interferometers have said first and second planes respectively; said second plane of said first interferometer and said reflected surface are arranged to oppose in parallel or said second plane of said first interferometer and said reflected surface are arranged obliquely and to oppose in such a manner that an imaginary plane including said second plane of said first interferometer and an imaginary plane including said reflected surface intersect in a line or at a point; and wherein said second plane of said second interferometer and said reflected surface are arranged to oppose in parallel or said second plane of said second interferometer and said reflected surface are arranged obliquely and to oppose in such a manner that an imaginary plane including said second plane of said second interferometer and an imaginary plane including said reflected surface intersect in a line or at a point.
 8. The optical dispersion compensator according to claim 7, wherein the reflection factor of said first plane of each of said first and second interferometers is 90% and over to 100% incl. while the reflection factor of said second plane of each of said first and second interferometers is 0% and over to 90% incl., and the reflection factor of the reflected surface of said mirror is 90% and over to 100% incl., and wherein the structure is arranged such that an optical beam is caused to become incident from between said second plane of said first interferometer and said reflected surface, said optical beam is alternately reflected between said interferometer and said mirror once, or two or more times, and thereafter said optical beam is caused to emit from between said second plane of said first interferometer and said reflected surface, said optical beam is caused to become incident from between said second plane of said second interferometer and said reflected surface, said optical beam is alternately reflected between said second interferometer and said mirror once, or two or more times, and thereafter, said optical beam is caused to emit from between said second plane of said second interferometer and said reflected surface, whereby wavelength dispersion is given to one or multiple signal light having different wavelength.
 9. The optical dispersion compensator according to claim 8, wherein the structure is arranged such that said second plane of each of said first and second interferometers has a square shape, that at least one of the reflection factor of said second plane of said first or second interferometer or thickness of said first or second interferometer is changed in a direction of a side of said square, and that each of said first and second interferometers is movable in parallel in said direction.
 10. The optical dispersion compensator according to claim 8, wherein there is provided temperature adjusting means for causing a change in temperature to at least one of said first or second interferometer adjacent to at least one of said first or second interferometer.
 11. The optical dispersion compensator according to claim 9, wherein for said mirror, there are 2n pieces of mirrors (n is a natural number) and for said interferometer, and there are 4n pieces of interferometers, and wherein n pieces of plus variable dispersion compensation units having a positive variable dispersion compensation function and n pieces of minus variable dispersion compensation units having a negative variable dispersion compensation function are connected in multistage.
 12. The optical dispersion compensator according to claim 10, wherein for said mirror, there are 2n pieces of mirrors (n is a natural number) and for said interferometer, there are 4n pieces of interferometers, and wherein n pieces of plus variable higher order dispersion compensation units having a positive variable higher order dispersion compensation function and n pieces of minus variable higher order dispersion compensation units having a negative variable higher order dispersion compensation function are connected in multistage.
 13. The optical dispersion compensator according to claim 7, wherein there are provided a plurality of said mirrors: a first mirror and a second mirror, said first and second mirrors have said first and second reflected surfaces respectively; and said second plane of said first interferometer and said first reflected surface of said first mirror are arranged to oppose in parallel or said second plane of said first interferometer and said first reflected surface of said first mirror are arranged obliquely and to oppose in such a manner that an imaginary plane including said second plane of said first interferometer and an imaginary plane including said first reflected surface of said first mirror intersect in a line or at a point; wherein said second reflected surface of said second mirror and said first reflected surface of said first mirror are arranged to oppose in parallel or said second reflected surface of said second mirror and said first reflected surface of said first mirror are arranged obliquely and to oppose in such a manner that an imaginary plane including said second reflected surface of said second mirror and an imaginary plane including said first reflected surface of said first mirror intersect in a line or at a point; and wherein said second plane of said second interferometer and said first reflected surface of said first mirror are arranged to oppose in parallel or said second plane of said second interferometer and said first reflected surface of said first mirror are arranged obliquely and to oppose in such a manner that an imaginary plane including said second plane of said second interferometer and an imaginary plane including said first reflected surface of said first mirror intersect in a line or at a point.
 14. The optical dispersion compensator according to claim 13, wherein the reflection factor of said first plane of each of said first and second interferometers is 90% and over to 100% incl. while the reflection factor of said second plane of each of said first and second interferometers is 0% and over to 90% incl., and the reflection factor of the reflected surface of each of said first and second mirrors is 90% and over to 100% incl., wherein the structure is arranged such that an optical beam is caused to become incident from between said second plane of said first interferometer and said reflected surface of said first mirror; said optical beam is alternately reflected between said interferometer and said mirror once, or two or more times, thereafter said optical beam is caused to emit from between said second plane of said first interferometer and said reflected surface of said first mirror; and said optical beam is reflected by the second reflected surface of said second mirror, and then, said optical beam is caused to emit; and said optical beam is caused to become incident from between said second plane of said second interferometer and said first reflected surface of said first mirror; said optical beam is alternately reflected once, or two or more times between said second plane of said second interferometer and said second reflected surface of said second mirror, and thereafter, said optical beam is caused to emit from between said second plane and said reflected surface of said second mirror, whereby wavelength dispersion is given to one or multiple signal light having different wavelength.
 15. The optical dispersion compensator according to claim 14, wherein the structure is arranged such that said second plane of each of said first and second interferometers has a square shape, that at least one of the reflection factor of said second plane of said first or second interferometer or thickness of said first or second interferometer is changed in a direction of a side of said square, and that each of said first and second interferometers and said second mirror is movable in parallel in said direction.
 16. The optical dispersion compensator according to claim 14, wherein there is provided temperature adjusting means for causing a change in temperature to at least one of said first or second interferometer adjacent to at least one of said first or second interferometer.
 17. The optical dispersion compensator according to claim 15, wherein for said mirror, there are 4n pieces of mirrors (n is a natural number) and for said interferometer, there are 4n pieces of interferometers, and wherein n pieces of plus variable dispersion compensation units having a positive variable dispersion compensation function and n pieces of minus variable dispersion compensation units having a negative variable dispersion compensation function are connected in multistage.
 18. The optical dispersion compensator according to claim 16, wherein for said mirror, there are 4n pieces of mirrors (n is a natural number) and for said interferometer, there are 4n pieces of interferometers, and wherein n pieces of plus variable higher order dispersion compensation units having a positive variable higher order dispersion compensation function and n pieces of minus variable higher order dispersion compensation units having a negative variable higher order dispersion compensation function are connected in multistage. 